Typical printing processes include four-color imaging units or modules having ink or toner of standard colors such as cyan, magenta, yellow and black which represent the CMYK color model. These four imaging units are capable of printing a color gamut that includes most of the colors that are required by customer print job orders.
When transparent toner is used as an insignia (or authentication mark) on top of the color image, the color below the transparent toner will be affected depending on the amount of clear toner laydown, paper substrate type, and amount of underlying color toners as well as the fusing condition. If the transparent toner is used over the whole color image, then a new color profile can be generated for that image under coverage by the transparent toner layer (with either full coverage or inverse mask coverage). However if the insignia is used only over a portion of the image, then the color below the transparent toner is different than that on the adjacent region without transparent toner when the same color profile is used for the color toner image. So not only is the gloss level of the insignia different than the image below, but also the color below the transparent toner is affected. In some cases, it is distracting for the viewer, who is supposed to see the insignia of different gloss if viewed at an angle different than the normal viewing angle of the image, to see the color change of the image under the insignia which is created for an authentication purpose or esthetic effect when viewed at a different angle. Therefore, it is desirable to create an insignia system with the transparent toner, so that the color under the transparent toner is similar to color adjacent to it. The transparent toner insignia can be created with a slightly different gloss level that can be visible when viewed at a non-normal viewing angle.
Various color models are referenced in the detailed description of the invention, so the following background is provided as a brief point of reference for the invention description. The RGB (red/green/blue) color model is based on adding light, i.e., colors are created by emitting red, green and blue beams of light. In the RGB color model, pixel colors can be changed by combining various values of red, blue and green. Each primary color has a range of values from 0 to 255. Thus, with 256 possible values of each primary color, the total number of possible colors is approximately 16.7 million. Printing technology uses a combination of cyan, magenta, yellow and black inks that reflect and absorb various wavelengths of light. Colors created by overprinting these four colors are part of the CMYK color model. The CMYK color model is the basis for four color process printing, to print continuous tone images on a commercial printer. In four color process printing, colors are reproduced on a commercial printer by using four different plates: cyan, magenta, yellow and black. By combining inks of cyan, magenta and yellow, a commercial printer can reproduce a significant portion of the visible spectrum of colors.
The Lab (or CIE Lab) color model provides a means for creating consistent, device-independent color, so that colors should not vary regardless of the monitor or printer used. The Lab color model is based on changes in a color's lightness (L values from 0 to 100), an “a” axis of colors ranging from green to magenta, and a “b” axis of colors ranging from blue to yellow. In the description that follows, perceptual space refers to the human perceptual space. The International Commission on Illumination (CIE Commission Internationale de l'Eclairage) has attempted to create a uniform color space so that an equal color difference (deltaE) in one color region can have a similar perceptual difference in another color region.
Color perception is a sensory perception triggered by the color in the brain of an observer. Describing the color has two components: a chromatic stimulus which is the measurable radiation reflected by an observed body; and a color stimulus specification which is the result of the visual perception in the eye of the observer.
Gloss is a perception based on the physical, optical property of a surface to reflect projected light more or less secularly. Gloss is a characteristic of the surface of a material. Toner is the color substance used in electrophotography. The toner particles are charged electrostatically in the developing unit and are attracted to the drum by the oppositely charged image portions of the photoconductor.
The gamut is a term in the art for the visible color range of a color model. For example, the gamut of RGB color model is larger than that of CMYK color module. The Lab color gamut encompasses the color gamuts of both RGB and CMYK.